A conventional wiper control system selects a wiping interval related to a wiping speed of a wiper and a wipe mode preset to a predetermined speed range. For example, an intermittent operation mode, a low-speed operation mode, and a high-speed operation mode. The selection is made in accordance with the amount of raindrops landed on a windshield so as to control a wipe operation of the wiper. This type of wiper control system includes a raindrop sensor for detecting the amount of raindrops landed on, for example, a windshield. The raindrop sensor is provided so as not to impair the visibility of a driver. As a result, a raindrop detection area is limited so as to be likely to generate fluctuations in the amount of landed raindrops measured by the raindrop sensor as compared with the amount of raindrops in actual rainfall. Therefore, it is necessary to stabilize the wipe operation.
On the other hand, in a technique disclosed in Japanese Patent Laid-Open Publication No. 2003-160026, the amount of landed raindrops is detected for each wipe cycle. Then, a wipe mode is selected based on an average amount of landed raindrops obtained by averaging the amounts of landed raindrops for a plurality of wipe cycles (hereinafter, referred to as a plurality of samples). If rainfall changes, the number of samples used for averaging is reduced. The reduction of the number of samples also reduces time until the actual rainfall is reflected on a calculated average amount of landed raindrops. As a result of this reduction of time, the ability to adjust the wipe operation in response to a change in a rainfall state is intended to be improved.
In the case where vehicle behavior changes, for example, when the vehicle starts after a stop at red light or when the vehicle decelerates to stop at red light, there is a possibility that the amount of raindrops landed on a windshield appears to suddenly change. In a conventional wiper control system, however, relatively long time is needed so that a suddenly changed amount of landed raindrops is reflected on the amount of landed raindrops for all samples even if the number of samples is reduced. Therefore, there arises a problem that the adjustment of the wipe operation is delayed following a sudden change of the amount of landed raindrops caused by a change in vehicle behavior and the like.
Even if the rainfall has no change, the amount of raindrops landed on the windshield is likely to suddenly increase in the case where the vehicle accelerates. For example, when the vehicle starts after a stop at a red light or when the vehicle overtakes a vehicle in front or the like after running at an approximately fixed speed and the like. On the other hand, the amount of raindrops landed on the windshield is likely to suddenly decrease in the case where the vehicle decelerates. For example, when the vehicle stops at a red light and the like.
Moreover, regarding a change in vehicle behavior, there is a possibility that it becomes difficult to ensure visibility until a wiping speed of the wiper is adjusted to be suitable for the amount of landed raindrops in the case of acceleration. In deceleration, for example, there is a possibility that the wipe operation mode of the wiper is kept to maintain a relatively high wiping speed although the amount of landed raindrops decreases. Therefore, the wipe operation of the wiper seems to be troublesome to a driver.